There is an increasing interest in applying devices for exploring the subsurface of the earth in a permanent or semi-permanent setting at respectively the earth's surface (which is herein intended to encompass any land surface or surface below a volume of water, for instance the seabed), above the earth formation of interest, for instance a reservoir containing hydrocarbon fluids (natural gas and/or oil). The devices are anchored at the surface or seabed and may provide a continuous stream of measurement data, which for instance can be used to monitor how the reservoir is depleted and which parts of a field need special attention to increase the overall recovery of the hydrocarbons.
An example of a device for exploring the subsurface is a geophone that converts seismic waves reflected from the formations in the subsurface into an electrical signal. Another example is a hydrophone that is a submersible device for converting the seismic waves reflected by the subsurface geology and causing pressure variations in the water into similar electrical signals.
While the geophones and hydrophones used to be placed at the surface of the land or at the seabed, it is believed that placing the geophones and hydrophones below the earth surface, for instance 30 m or deeper below surface, may cause the quality of the measured seismic data to improve markedly. The poor quality of the seismic signals measured at the surface may be caused by the influence of surface noise and acoustically poorly defined top soils. This influence is not present or less so in seismic signals measured sufficiently below the surface of the earth.
A geophone normally comprises one or more transducers for converting the vibrations in one or more directions into corresponding electrical signals. The transducers are placed in a housing and further includes support means, such as a base, ground plate, a spike of gimbal for supporting the housing in an operative position with respect to the earth's surface. However, for obvious reasons it is difficult to properly arrange geophones with such support means at a position below the earth surface. If for instance horizontal boreholes are made in the subsurface and the geophones (or hydrophones) are to be placed in the boreholes, the geophones cannot be fixed to the earth in the usual way. One may be able to lower the geophones into the borehole, but arranging them at exactly the right positions and fixing them to the wall of the hole or to the casing provided inside the borehole turns out to be difficult if not impossible. If the geophones can neither be properly positioned in the borehole nor be firmly fixed to the borehole, this may have an adverse influence on the signal to noise ratio of the geophones.